1. Field of the Invention
The present invention relates to an image sensor, and more particularly, to an image sensor using a back-illuminated photodiode which absorbs light through its bottom side and a method of manufacturing the image sensor.
2. Description of the Related Art
A complementary metal-oxide semiconductor (CMOS) image sensor is a device for measuring light intensity. In general, such image sensor has a structure in which a photodiode is formed on a plane where a digital/analog circuit is formed, and a metal wire and an inter-metal dielectric (IMD) layer are formed on a top side of the photodiode.
In the conventional image sensor, light passes through a micro-lens and a color filter and thereafter proceeds along a pathway of several films before being absorbed by the photodiode. Throughout this process, sensitivity of light, particularly, blue light, deteriorates.
Furthermore, in the conventional image sensor, after passing through green, red, and blue color filters, light is reflected by a multi-layered metal wire. This affects an adjacent photodiode, resulting in a color crosstalk. Also, an electrical crosstalk occurs in a bottom side of the photodiode due to red light having a long wavelength.
In the image sensor using the back-illuminated photodiode, instead of passing through the IMD layer laminated on the top side of the photodiode, light is directly absorbed by a silicon substrate on which the photodiode is formed. Thus, sensitivity of light is significantly improved.
In order to employ a structure of the back-illuminated photodiode, there is a need to remove noise caused by a surface defect, for example, a dangling bond existing on a surface of an epitaxial layer with low concentration. However, there has been a problem in that heat treatment cannot be performed at a high-temperature because of the existing metal wire.
Therefore, the conventional image sensor using the back-illuminated photodiode cannot solve a noise problem that is resulted from a defected surface of a silicon substrate on which the back-illuminated photodiode is formed.
To address this problem, laser scanning is generally used for instantaneous high-temperature heat treatment, which is disadvantageously expensive and time consuming.